Pretty ugly.
- posted
7 years ago
-- John Larkin Highland Technology, Inc picosecond timing precision measurement
Pretty ugly.
-- John Larkin Highland Technology, Inc picosecond timing precision measurement
Brushless DC?
M
Yup. 24 volts, 14 CFM. If you stall the blade, it shuts down to maybe
20 mA and, every few seconds, tries briefly to start up again, at roughly 90 mA for 300 ms.-- John Larkin Highland Technology, Inc picosecond timing precision measurement
I wonder if that's typical for all brushless fans, or if that one's special.
-- Tim Wescott Wescott Design Services
I have seen other BLDC fans squirt a lot of ripple into power rails. I think it's pretty typical.
I'm going to drive this fan from a DAC and an opamp, so I think I'll add some passive filtering between the amp and the fan. Current is modest, so an RC should do.
Just tried a tiny 12V Sunon fan, and it's equally ugly.
-- John Larkin Highland Technology, Inc picosecond timing precision measurement
Doesn't quite look like what I observed in the early '80's, but similar.
In our (OmniComp/GenRad) case, the current notch was causing "burbles" on the CRT display.
My solution was to devise a "fill-in" circuit such that the current draw was flat. ...Jim Thompson
-- | James E.Thompson | mens | | Analog Innovations | et |
Fan_Current.JPG
If you really want to isolate the fan from the power rail, make the final drive a collector or drain of a transistor, put a honkin' big cap in parallel with the fan, and make the op-amp stage slow to respond to changes in fan voltage. I'm confident that you can make it stable (if slow), but I haven't bothered to even start to think about how big a cap it needs to be, so I may have my head buried firmly up my ass.
-- Tim Wescott Wescott Design Services
"Honkin' big" turns out to be too honkin', my solution...
Message-ID:
was down-right trivial. Sense the fan current and make a loop that makes the current draw constant. ...Jim Thompson
-- | James E.Thompson | mens | | Analog Innovations | et |
You'd want to test to see how wild the voltage swings end up, and whether the fan still works. But -- maybe.
-- Tim Wescott Wescott Design Services
VERY typical. Many of them are two phase motors, and some use PWM to control speed. They use the simplest control schemes imaginable, some don't even use a chip, just a couple transistors tied to Hall sensors. I admit I haven't opened any up in some time, so the tech may have advanced a bit.
Jon
Right. I have half of a monster opamp (TCA0372) available. That will run off +24 and power the fan to ground. 10 ohms and 220 uF should isolate the fan from everything else pretty well. I don't expect to need all the available CFMs to cool the box, so a little DC voltage drop is not a problem.
The uP can sense board temperature and gently tweak the fan speed, nothing too obvious. Slew-limited bang-bang control maybe. We'll even wind up pushing the board more toward constant temperature, which will improve the specs a little.
-- John Larkin Highland Technology, Inc picosecond timing precision measurement
It's in the OmniComp/GenRad portable board tester, circa 1980-83.
I think you missed the point... it makes the power supply draw constant, not the fan current... thus the noise component is reduced significantly... proven by observation... no CRT "burbles" >:-} ...Jim Thompson
-- | James E.Thompson | mens | | Analog Innovations | et |
Fan_Current.JPG
All cheap DC fans use an ASIC with Hall sensor and two open-collector outputs. With this topology, they can't use diodes for recirculation. The current drops to zero during commutation. Better quality fans have a full-bridge driver and much lower current spikes. I spent some time working on lower noise fan drivers, but nobody was really interested. The chip cost is less than $0.05 the last I heard.
Fan_Current.JPG
I was surprised to find on some recent PWM BLDC fans that they don't actually use the PWM signal to gate the current directly. Instead they measure the PWM duty cycle and use that to control the fan speed.
Other behaviour I've noticed on this type of fan:
- there's a delay between changing the PWM duty cycle and a change in fan speed.
- there can be overshoot in fan speed to a step change in PWM duty cycle.
I guess these things wouldn't matter too much unless the fan was inside some sort of feedback loop.
Regards, Allan
Do you know any particular models? I'd like to see who's making the driver chip.
We tried PWMing the DC power to our 24 volt BLDC fan, and that didn't work. A switcher, with a PWM'd mosfet, inductor, catch diode, cap, was messy because the inductor L would have to be huge.
So I'll control it linear, with a giant opamp.
-- John Larkin Highland Technology, Inc lunatic fringe electronics
n .
I've used a standard switcher and adjusted the voltage by feeding current into the divider that set the voltage
-Lasse
so leave out the L&C.
NT
What does that 'sine wave only' button do? I'd like to have a digital scope but they seem to be getting weirder.
[snip]
Sort of like this, as best I can remember from 30+ years ago...
The problem at hand was that the fan ripple current was magnetically coupling to the CRT (portable board tester instrument the size of a thin briefcase), with no room to properly fix it.
My kluge reduced the ripple current dramatically ;-) ...Jim Thompson
-- | James E.Thompson | mens | | Analog Innovations | et |
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